Abstract:

A silica glass crucible used for pulling single-crystal silicon, which
includes a cylindrical straight body part, a bottom part and a curved
part located between the straight body part and the bottom part, wherein
the curvature radius of the inner wall surface of the curved part is 100
to 240 mm. Variation of the wall thickness W of the curved part is
preferably 0.1 to 1.4 mm/cm.

Claims:

1. A method for pulling single-crystal silicon comprising:melting
polycrystalline silicon in a silica glass crucible, the silica glass
crucible comprising a cylindrical straight body part, a bottom part and
the curved part located between the straight body part and the bottom
part, wherein a curvature radius of the inner wall surface of the curved
part is 100 to 240 mm;immersing a seed composed of single-crystal silicon
in the molten polycrystalline silicon; andforming a single-crystal
silicon ingot by pulling the seed while rotating the silica glass
crucible.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a divisional of U.S. patent application Ser. No.
12/251,614, filed Oct. 15, 2008, the content of which is expressly
incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

[0002]1. Technical Field

[0003]The present invention relates to a silica glass crucible which is
used for pulling single-crystal silicon, and a method for pulling
single-crystal silicon.

[0004]2. Background Art

[0005]Single-crystal silicon which is used for a semiconductor material is
mainly manufactured by pulling up from molten polycrystalline silicon,
and a silica glass crucible is used for holding a polycrystalline silicon
melt. The silica glass crucible is put in a carbon susceptor and heated
from the high temperature surroundings. In order to uniformly transfer
this heat, a silica glass crucible whose outer layer is a translucent
layer having bubbles and whose inner surface is a transparent layer
having substantially no bubbles, is disclosed in Patent Document 1:
Japanese Unexamined Patent Publication No. 06-92779. However, in the case
of manufacturing a silica glass crucible by a rotary mold method, silica
powder which has accumulated on the inner surface of the mold is heated
and melted, vitrified silica is subjected to downward force due to its
own weight, and as a result, the curved part of the crucible tends to
grow in thickness. Consequently, there is a possibility that the heat
transfer at a curved part of the crucible differs from the surrounding
area thereby becoming less uniform.

[0006]Meanwhile, with regard to the curved part of a silica glass
crucible, when a crucible is used under high temperature for a long
period of time, the glass strength is gradually lowered, a curved part of
the crucible is subjected to intense local stress due to its own weight
and then deforms to decrease the yield of single crystal in some cases.
In order to prevent this, a silica glass crucible in which the strength
of a curved part of the crucible is enhanced by the setting wall
thickness of the curved part as 1.5 to 1.8 times thicker than that of the
straight body part of the crucible is disclosed in Patent Document 2: WO
02/014587. However, since a lower part below the curved part of the
crucible is within a range where the surface area of a silicon melt
gradually decreases when the liquid level of the silicon melt is lowered
due to the pulling of a single-crystal silicon, there is a problem that
silicon being pulled becomes polycrystalline when the surface area of
this liquid level rapidly decreases. Thus, although the curved part of a
crucible is thickened, if a curvature radius of the curved part is
inappropriate, the yield of single crystal cannot be increased.

[0007]The present invention has resolved the above-mentioned Conventional
problems in a silica glass crucible for pulling single-crystal silicon,
and provides a silica glass crucible which can suppress
polycrystallization upon pulling single-crystal silicon and increase the
yield of single crystal by being adjusted in a curvature radius of the
curved part of the crucible, more preferably the change of wall thickness
to a predetermined range.

SUMMARY OF THE INVENTION

[0008]The present invention relates to a silica glass crucible and a
method for pulling single-crystal silicon, which include any one of the
constitutions described below in order to solve the above-mentioned
problems,

[0009](1) A silica glass crucible which is used for pulling single-crystal
silicon is characterized in that the curvature radius R1 of the inner
wall surface of a crucible curved part is 100 to 240 mm. Here, the
crucible curved part refers to a curved region which has a relatively
small curvature radius and is an area between a cylindrical straight body
part and a bottom part having a large curvature radius. Furthermore, the
curvature radius of the crucible curved part is the minimum curvature
radius of an inner peripheral surface of the curved part.

[0010](2) The silica glass crucible described in the above (1), in which
variation of the wall thickness W of the crucible curved part is 0.1 to
1.4 mm/cm. Here, the variation of wall thickness W refers to the amount
of change in wall thickness W when a measurement location is moved in a
direction along an axis of the crucible.

[0011](3) The silica glass crucible described in the above (1), in which
variation of the wall thickness W of the curved part is 0.2 to 0.5 mm/cm.

[0012](4) A method for pulling single-crystal silicon using the silica
glass crucible described in any one of the above (1) to (3), which
includes the steps of melting polycrystalline silicon in the silica glass
crucible; immersing a seed composed of single-crystal silicon in the
molten silicon; and forming a single-crystal silicon ingot by pulling the
seed while rotating the silica glass crucible.

[0013]For the silica glass crucible of the invention, the curvature radius
of the curved part is adjusted to be within a predetermined range.
Therefore, the surface area of the liquid is slowly reduced when the
liquid level of molten silicon is lowered along the curved part in a
process of pulling up a single-silicon crystal ingot, and thus a rapid
change of the surface area hardly occurs. Moreover, when change of the
wall thickness of the curved part is adjusted within a predetermined
range, temperature distribution in the inner peripheral surface of the
curved part becomes uniform. Consequently, silicon is difficult to
polycrystallize and the yield of a single crystal can be increased upon
the pulling single-crystal silicon.

[0014]"Silica" in the specification is not limited to general silica and
includes any material known as a raw material for a silica glass
crucible, such as silicon dioxide (silica), crystal and silica sand.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a longitudinal sectional view showing one embodiment of a
silica glass crucible related to the invention.

[0016]FIG. 2 is a longitudinal sectional view showing a state for pulling
up a single-crystal silicon ingot being pulled from a silicon melt in a
silica glass crucible of one embodiment.

[0017]FIG. 3 is a longitudinal sectional view showing another embodiment
of the invention.

[0019]Hereinafter, the present invention will be illustrated in detail
with reference to drawings.

[0020]FIG. 1 is a longitudinal sectional view showing one embodiment 11 of
a silica glass crucible related to the invention, and FIG. 2 is a
longitudinal sectional view showing a single-crystal silicon ingot I
being pull up from a silicon melt Yin a silica glass crucible 11. S1 and
S2 each show the liquid level of the silicon melt Y.

[0021]The silica glass crucible 11 of this embodiment is used for pulling
single-crystal silicon, and characterized in that the crucible has a
straight body part 13, a bottom part 14 and a curved part 12 which is
located therebetween as shown in the figure, and a curvature radius R1 of
the inner wall surface of this curved part 12 is 100 to 240 mm. The
straight body part 13 of the crucible is a cylindrical part having an
axis C of the crucible, and extends upward from the curved part 12 as
shown in the figure.

[0022]The curved part 12 of the crucible starts from the bottom end of the
straight body part 13 to a point touching the bottom part 14 as shown in
the figure. The curvature radius R1 of the inner wall surface of the
curved part coincides with the curvature radius R2 of the inner wall
surface at the bottom part at the boundary between the curved part 12 and
the bottom part 14. In this embodiment, a central point M1 of the
curvature radius R1 of the curved part 12 lies on the horizontal line
extending from the bottom end of the straight body part 13 (that is, top
end of the curved part 12) as shown in FIG. 1. Meanwhile, a central point
M2 of the curvature radius R2 of the bottom part 14 lies adjacent to a
point at the intersection of a center line of the crucible with a top end
of the crucible.

[0023]In the case where a radius of the crucible is between 22 and 32
inches, the minimum curvature radius R1 of the curved part 12 is
preferably in the range of 100 to 240 mm, and the maximum curvature
radius R2 of the bottom part 14 is largely in the range of 550 to 900 mm.

[0024]Where the curvature radius R1 of the curved part 12 is smaller than
100 mm, inclination of the curved part 12 (an angle with respect to a
horizontal plane) changes rapidly. Accordingly, as shown in FIG. 3, when
a liquid level S1 of a silicon melt inside the crucible is gradually
lowered with progress of pulling up the seed and reaches a liquid level
S2 coming in contact with the curved part 12, and an area As 2 of the
liquid level S2 rapidly becomes smaller than an area As 1 coming in
contact with the straight body part 13, which thus has an affect on a
silicon solid-liquid interface thereby readily causing
polycrystallization of silicon.

[0025]Alternatively, when the curvature radius R1 of the curved part is
more than 240 mm, it is not substantially different from the bottom part
14, which thus leads to a rapid change in an angle of the boundary
between the straight body part 13 and the curved part 12. As a result,
stress, such as the weight of the crucible, easily localized, thus it is
not preferable from the viewpoint of enhancing the strength of the
crucible.

[0026]More preferably, the minimum curvature radius R1 of the curved part
12 is 12 to 45% of an inside diameter of the crucible D, and the
curvature radius R2 of the bottom part 14 is 60 to 220% of an inside
diameter D of the crucible. Further preferably, the minimum curvature
radius R1 of the curved part 12 is 15 to 35% of an inside diameter D of
the crucible, and the curvature radius R2 of the bottom part 14 is 80 to
150% of an inside diameter D of the crucible.

[0027]Moreover, in a silica glass crucible 11 of this embodiment,
variation of wall thickness W of the curved part 12 (the difference in
wall thickness/distance between inside surfaces) is 0.1 to 1.4 mm/cm, and
preferably 0.2 to 0.5 mm/cm. This variation of wall thickness refers to a
value indicating a difference between the thickness of each measurement
spot located within a range from a top end of the curved part 12, that is
a boundary of the straight body part 13, to a boundary of the bottom part
14, and the thickness of each measurement spot located along a crucible
axis line C. For example, when the wall thickness of the curved part 12
at the measurement spot X1 is W1, and the wall thickness at the
measurement spot X2 distanced by a predetermined distance L in a
direction along the crucible axis line C is W2, a value obtained by
dividing the thickness difference (W1-W2) by the distance L is the wall
thickness variation W [W=(W1-W2)/L].

[0028]When the variation of wall thickness is smaller than 0.1 mm/cm, the
wall thickness of the curved part 12 becomes insufficient and a crucible
11 may deform. Alternatively, when the variation of this wall thickness
is larger than 1.4 mm/cm, the difference in the wall thickness of the
crucible becomes larger and heat distribution changes rapidly. Therefore,
polycrystallization of silicon readily occurs, and thus the yield of
single crystal decreases. The wall thickness is measured on the basis of
a cross section line perpendicular to the inner wall surface.

[0029]The maximum wall thickness of the curved part 12 of the crucible is
preferably one to three time(s) thicker than the average wall thickness
of the straight body part 13 of the crucible, and more preferably 1.1 to
2 times. In this case, the strength of the curved part 12 of the crucible
can be further enhanced.

[0030]According to the silica glass crucible 11 of this embodiment, since
the change in a surface area of the liquid level of a silicon melt
becomes moderate upon pulling a single-crystal silicon ingot I as shown
in FIG. 2 and the heat distribution of the curved part 12 is also
uniform, polycrystallization of silicon hardly occurs and the yield of
single crystal can be increased.

[0031]In order to pull single-crystal silicon using the silica glass
crucible 11 of this embodiment, polycrystalline silicon is melted in the
silica glass crucible 11, a seed composed of single-crystal silicon (not
shown in FIGS.) is immersed in a silicon melt Y, and the seed is pulled
up while the silica glass crucible 11 is rotated around an axis of the
crucible C, thereby forming a single-crystal silicon ingot I.

[0032]According to such a method for pulling single-crystal silicon, use
of the silica glass crucible 11 provides the advantage that a rapid
change of the heat distribution of the curved part of the crucible and
the liquid level can be suppressed, polycrystallization when pulling
single-crystal silicon in a silicon melt inside the crucible can be
suppressed and the yield of single crystal can be increased.

[0033]FIG. 3 shows another embodiment of the invention. The silica glass
crucible 11 of this embodiment includes an outer layer 22 which is a
translucent layer having bubbles and an inner layer 20 which is a
transparent layer having substantially no bubbles. Other constitutions
are the same with the foregoing embodiments. The present invention can
also be applied to a crucible having such a double-layered structure.

Examples

[0034]Hereinafter, Examples and Comparative Examples according to the
invention will be demonstrated together.

[0035]Single-crystal silicon was pulled up using a silica glass crucible
(32 inches in diameter) as shown in FIG. 1. The results are shown in
Table 1.

[0036]When a silica crucible (No. 2 or No. 3) in which the curvature
radius R1 of the inner surface of the curved part 12 and a wall thickness
variation W of the curved part 12 are within the range of the invention
is used, the yield of the single crystal was between 78 and 83% and the
pulling time was 78 to 83 hours.

[0037]On the other hand, in the case of comparative specimens (No. 1 and
No. 4) whose curvature radius R1 of the inner surface was less than 100
mm or more than 240 mm, the yield of the single crystal was significantly
low (as low as 40%) because polycrystallization of silicon occurred when
the hot water level of a silicon melt reached the curved part R1.

[0038]Moreover, with the silica glass crucibles used for these comparative
specimens, when silicon was polycrystallized in the silica glass
crucible, an operation of re-pulling single crystal by melting the
polycrystallized part of silicon had to be repeated. Thus, the pulling
operation took longer than necessary in total by carrying out a
re-melting operation, meaning that the pulling period was extended to a
large extent. In addition, the pulling has to be performed within the
scope of durability, and the limit of the total pulling-up time is
approximately 150 hours. The pulling time in the comparative specimen No.
4 was close to the limit.

[0039]Hereinbefore, preferred embodiments of the invention have been
illustrated, but the invention is not limited thereto. Additions,
omissions, substitutions, and other modifications can be made without
departing from the spirit or scope of the invention. Accordingly, the
invention is not to be considered as being limited by the foregoing
description, and is only limited by the scope of the appended claims.